Inorganic particles have a wide variety of uses as pigments or as additives to pigments that are important in the manufacture of paints, plastics, paper, laminates, coatings and other applications.
There have been significant efforts to produce inorganic pigments with desired characteristics (i.e., particle size, shape, coating and/or crystallinity) for the particular application. The prior art mixes, blends, or coats inorganic base particles with other inorganic solids to obtain compositions with useful properties such as brightness, opacity, and light scattering. Typically, insufficient mechanical dispersion of the base particle and other inorganic solids causes the composition to flocculate or agglomerate reducing, among other things, light-scattering, brightness and opacity. The tendency to flocculate or agglomerate can also be reduced by coating the inorganic base particles with suitable inorganic solids that effect the surface charge and other properties.
One particularly useful inorganic base particle comprises titanium dioxide. Titanium dioxide base particles are often incorporated into paint, plastic, paper, or other coating systems. This pigment imparts desirable properties such as, for example, brightness, opacity, light scattering, and durability.
It is known that light-scattering characteristics of pigments do not improve greatly above a certain pigment concentration. One reason light-scattering efficiency is decreased at high pigment concentrations is an effect known as xe2x80x9ccrowding.xe2x80x9d
An inorganic pigment particle of optimum size may have a light scattering cross-section that is 3-4 times larger than the projected cross-section (area) of the pigment particle. At high pigment concentrations, where the pigment particles are close to each other, overlapping of light scattering cross-sections causes a severe reduction in the pigment""s scattering efficiency. For example, in order to avoid crowding in a TiO2-based pigment when used in paper, the distance between the TiO2 pigment particles should be kept to about 0.1 xcexcm or more apart.
The crowding effect becomes even more serious when pigment particles are blended or mixed, which often causes agglomeration of the particles in the resultant composition. This effect is particularly seen in some papermaking processes.
There are other factors that can affect the scattering efficiency of pigment mixtures with high pigment particle concentration, especially in papermaking processes. Scattering efficiency in these processes depends strongly on the medium in which the pigment particle is incorporated. For example, the scattering efficiency of pigment particles dispersed in air can be 2 to 3 times higher than the same pigment particles dispersed in resins or fillers.
In order to reduce the crowding effect, it is common to add an extender, also called a spacer or filler, to the pigment in order to space apart the pigment particles. Some spacers and fillers include calcium carbonate, clay, silica, alumina, and other metal oxide particles. Typically, spacers and fillers have a lower cost than the base inorganic solid (i.e., titanium dioxide) and therefore reduce the cost of the resultant pigment composition.
Considerable research effort has been directed to making pigment compositions having reduced amounts of base particles. There have been attempts to blend or mix base particles with other less expensive inorganic solids resulting in spaced pigment products. However, the resultant products from blending or mixing have poor light-scattering, brightness and opacity characteristics. For example, when clay is mechanically mixed or blended with titanium dioxide, it is not possible to control the distribution of clay spacer particles throughout the mixture uniformly because the clay particles do not attach effectively to the surfaces of the titanium dioxide particles. In addition, blending methods are especially susceptible to pigment particle agglomeration, which reduces light scattering characteristics of the pigment product.
Other methods of coating pigment particles seek to achieve better dispersion by forming the spacer particles in situ on the pigment particles. For example, precipitated calcium carbonate (PCC) spacer particles are deposited on the surface of titanium dioxide base particles by bubbling carbon dioxide through a slurry containing lime and titanium dioxide. This precipitation method achieves the making of PCC spacers, but does not provide proper control over the precipitated particle size. Common problems include insufficient size of the precipitated spacers and agglomeration of the spacers. These problems are due in part to the complexity of many factors that influence the formation of the spacers, such as reactant concentrations, additives, temperature, processing pH, ageing periods.
Based on the foregoing, there remains a need for methods and compositions with desired characteristics (i.e., particle size, shape, coating, or crystallinity) for end use applications. There is also a need for inorganic pigment methods and compositions with minimal agglomeration and improved light-scattering efficiency, brightness and/or opacity.
The present invention provides methods and compositions for changing the characteristics of pigments by precipitating inorganic solids in the presence of a suitable surfactant. The present invention provides compositions and methods that are cost-effective and produce inorganic compositions with minimal agglomeration and improved light-scattering efficiency, brightness and/or opacity. It has also been found that inorganic particles precipitated on the base particle can have substantially spherical shape and be a particular size that improves the light-scattering efficiency, brightness and/or opacity of the pigment composition.
In one embodiment, the present invention provides a pigment composition comprising one or more inorganic base particles and one or more substantially spherical shaped spacer particles deposited thereon, the pigment composition having a particle size range between about 0.1 and about 0.5 microns.
In another embodiment, the present invention provides a pigment composition comprising one or more titanium dioxide base particles, one or more substantially spherical shaped calcium carbonate particles deposited thereon and a surfactant, the pigment composition having a particle size range between about 0.1 and about 0.5 microns.
In yet another embodiment of the invention, an inorganic composition comprising one or more substantially spherical shaped inorganic particles, the particles having a size range between about 0.1 and about 1 microns is provided.
In one exemplary embodiment, the present invention provides an inorganic composition comprising one or more substantially spherical shaped calcium carbonate particles, the particles having a size range between about 0.1 and about 1 microns.
In another exemplary embodiment, of the present invention provides a method of preparing pigment particles comprising the steps of: preparing an aqueous slurry of base pigment particles, adding a surfactant and one or more than one spacer precursor to the slurry, and precipitating the spacer precursor on the base pigment particles under conditions so as to form substantially spherical shaped spacer particles on the base pigment particles, the base pigment particles having a particle size range between about 0.1 and about 0.5 microns.
In yet another exemplary embodiment, the present invention provides a method of making substantially spherical calcium carbonate particles comprising adding a surfactant to a source of calcium carbonate to obtain substantially spherical calcium carbonate particles.
In a preferred embodiment, the present invention provides a method of making substantially spherical calcium carbonate particles comprising adding carbon dioxide to a mixture comprising lime, water, and a surfactant to obtain substantially spherical calcium carbonate particles.
In a further embodiment, the present invention provides a pigment composition comprising one or more titanium dioxide base particles, one or more substantially spherical shaped silica particles deposited thereon and a surfactant, the silica having a particle size range between about 5 and about 50 nanometers.
In yet another preferred embodiment, the present invention provides a method of preparing titanium dioxide pigment particles comprising the steps of: preparing an aqueous slurry of base titanium dioxide, adding a surfactant and a source of calcium carbonate to the slurry, and precipitating calcium carbonate on the base titanium dioxide particles under conditions so as to form substantially spherical shaped calcium carbonate particles on the titanium dioxide particle, wherein the titanium dioxide particle has a particle size range between about 0.1 and about 0.5 microns.
In alternate embodiment, the present invention provides a method of preparing pigment particles comprising the steps of: preparing an aqueous slurry of base titanium dioxide pigment particles, adding a surfactant and one or more silica spacer precursors to the slurry, and precipitating the one or more silica spacer precursors on the base pigment particles under conditions so as to form substantially spherical shaped silica spacer particles on the base pigment particles, the base pigment particles having a particle size range between about 0.1 and about 0.5 microns.
In yet another alternate embodiment, the present invention provides paints, plastics and paper comprising the pigments of the present invention.
In other embodiments, the present invention provides paints, plastics and paper comprising the pigment made by the methods of the present invention.